A typical gas turbine engine has an annular axially extending flow path for conducting working fluid sequentially through a compressor section, a combustion section, and a turbine section. The compressor section includes a plurality of rotating blades which add energy to the working fluid. The working fluid exits the compressor section and enters the combustion section. Fuel is mixed with the compressed working fluid and the mixture is ignited to add more energy to the working fluid. The resulting products of combustion are then expanded through the turbine section. The turbine section includes another plurality of rotating blades which extract energy from the expanding fluid. A portion of this extracted energy is transferred back to the compressor section via a rotor shaft interconnecting the compressor section and turbine section. The remainder of the energy extracted may be used for other functions.
Efficient transfer of energy between the working fluid and the compressor and turbine sections is dependant upon many parameters. One of these is the orientation of the rotating airfoil relative to the flow direction of the working fluid. For this reason, a stage of non-rotating airfoils, referred to as vanes, are typically located upstream of a rotor blade stage. The vanes properly orient the flow for engagement with the blades. Another parameter is the size and shape of the airfoils, both blades and vanes. Typically the airfoils are aerodynamically optimized to efficiently transfer energy. Practical considerations, however, may restrict the size and shape to within certain constraints.
The amount of energy produced by the combustion process is proportional to the temperature of the combustion process. For a given fuel and oxidant, an increase in the energy of combustion results in an increase in the temperature of the products of combustion. The allowable temperature of the working fluid flowing through the turbine section, however, typically provides a temperature limit for the combustion process.
One method to prevent overheating turbine components is to cool the turbine section using cooling fluid drawn from the compressor section. Typically this is fluid which bypasses the combustion process and is thereby at a much lower temperature than the working fluid in the turbine section. The cooling fluid is flowed through and around various structure within the turbine section. A portion of the cooling fluid is flowed through the turbine airfoils, which have internal passageways for the passage of cooling fluid. As the cooling fluid passes through these passageways, heat is transferred from the turbine airfoil surfaces to the cooling fluid.
A detrimental result of using compressor fluid to cool the turbine section is a lower overall efficiency for the gas turbine engine. Since a portion of the compressed fluid is bypassing various stages of the turbine section, there is no transfer of useful energy from the compressor fluid to the bypassed turbine stages. The loss of efficiency is balanced against the higher combustion temperatures which can be achieved by cooling with compressor fluid. This balancing emphasizes the need to efficiently utilize the cooling fluid drawn from the compressor section. Efficient utilization of cooling fluid requires getting maximum heat transfer from a minimal amount of cooling fluid.
A common method of cooling a turbine vane utilizes an impingement tube or baffle disposed within the turbine vane. The baffle extends through the turbine vane and is in fluid communication with the source of cooling fluid. The baffle includes a plurality of impingement holes spaced about through which the cooling fluid passes. The cooling fluid exiting the baffle impinges upon the internal surfaces of the turbine vane. The arrangement of impingement holes distributes the cooling fluid within the turbine vane to prevent a deficiency in cooling from occurring in a particular location.
A drawback to using baffles is that the baffles present a limitation on the size and shape of the airfoil. First, the airfoil must be thick enough to permit insertion of the baffle within the airfoil. Second, complex shapes having 3-dimensional curvature are not practical as a result of having to insert the baffle into the airfoil.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop efficient turbine airfoil cooling to maximize the overall efficiency of a turbomachine with minimal impact upon aerodynamic shape of the turbine airfoil.